In certain magnetic recording disk files which have a relatively high density of data tracks, it is necessary to provide servo control of the position of the read/write heads to both maintain the heads over the tracks in the presence of various disturbances and to position the heads quickly and accurately to other tracks for subsequent read and write operations. In order to accomplish this, certain disk files utilize prerecorded servo signals on either a dedicated servo disk or on the data disks to provide a head position error signal (PES) to the head position control system. One method of incorporating this servo information is to record the information on a separate dedicated disk and use a servo read head on that disk to imply the location of read/write heads on the same actuator on other data disks within a given tolerance limit. In this method, the control system receives a continuous PES which permits very accurate positioning of the servo head, but the read/write heads suffer track misalignment due to thermal and mechanical effects, such as disk flutter.
In order to provide improved track registration of the heads, another method of incorporating the servo information is to embed prerecorded servo signals on the data disks in equally angularly spaced sectors which extend out radially from the data disk centers. As such a disk rotates, the read/write head, or a dedicated servo read head, receives sampled position signals as the servo sectors pass beneath the head. Because in this method the sectors of servo information are recorded on the disk between sectors of data, it is desirable to minimize the number and angular extent of these servo sectors in order to maximize the amount of disk surface area available for data. Various methods of head positioning using sector servo are described in assignee's U.S. Pat. Nos. 3,185,972; 4,297,734; 4,488,189; and 4,511,938. While there are numerous types of servo patterns which may be used with sector servo, two specific types are the "null" and "quad-burst" patterns described in the IBM Technical Disclosure Bulletin (TDB) articles of January 1976 (pp. 2656-2657) and July 1978 (pp. 804-805), respectively.
In commercially available disks, such as those which may be used in disk files using a sector servo method, the magnetic layer formed on the disk substrate is either a continuous thin film of magnetic metal alloy or metal oxide or a continuous film of organic material containing magnetic particles. Thus, during operation of the disk file, data may be recorded at any radial position on the disk where the continuous magnetic film is formed. Because the read/write head cannot always be precisely aligned over any predetermined radial position on the disk, new data recorded over a previously recorded track may be recorded on the disk in a radial position slightly offset from the track where the previous data was recorded. This may occur even though the previous data and the new data are identified as having been recorded on the same data track. When the read/write head reads the signal from this track it will also pick up previously recorded signals on the disk at the edges of the track. An additional problem with the use of a continuous magnetic film is that because the magnetic media extends radially on the disk on both sides of the read/write head, fringe magnetic fields from the read/write head during recording will generate magnetic patterns at the edges of the tracks. In the case of thin film metal alloy or metal oxide disks, this creates disorientation of the magnetic domains in the film at the track edges which results in noise when the recorded signal is read back.
In order to overcome the above shortcomings of disks with continuous magnetic films, disks with discrete magnetic tracks separated by discrete nonmagnetic guard bands or separator tracks have been proposed. One such disk and its fabrication process are described in assignee's concurrently-filed copending application, Ser. No. 07/187,110, filed Apr. 28, 1988, entitled "Thin Film Magnetic Recording Disk and Fabrication Process". While discrete track disks have been proposed, no such disks are commercially available. In addition, no successful servo technique has been proposed which takes full advantage of all the benefits which discrete track disks offer.